WASHINGTON, Sept. 1 — Iraqi casualties soared by more than 50 percent during the roughly three-month period ending in early August, the product of spiraling sectarian clashes and a Sunni-based insurgency that remains “potent and viable,” the Pentagon noted today in an comprehensive assessment of security in Iraq.

In a grim 63-page report, the Pentagon chronicled bad news on a variety of fronts. One telling indicator was the number of weekly attacks, which reached an all-time high in July.

The American-led coalition suffered the brunt of the attacks, but an increasing number are being directed against civilians. In Baghdad, for example, civilian targets accounted for 22 percent of all the attacks, up from 15 percent in April. And the attacks on Iraqi troops and civilians caused many more deaths than did those on American troops.

“Death squads and terrorists are locked in mutually reinforcing cycles of sectarian strife, with Sunni and Shia extremists each portraying themselves as the defenders of their respective sectarian groups,” the report noted. “The Sunni Arab insurgence remains potent and viable.”

The Pentagon report on “Measuring Security and Stability in Iraq” is mandated by Congress and issued quarterly. It covers a broad range of subjects, ranging from the economy to public attitudes to the training of Iraqi security forces.

This time, the study has been the focus of special interest because of increasing fears that Iraq is sliding into civil war. And its grimmer notes, echoing recent Congressional testimony by military commanders, come at a time when President Bush and members of his cabinet have been trying to present a strong case in support of the war, in the face of vehement criticism from Democrats.